Toss canceling, which is the ability to cancel landing lag by throwing an item.

I'm guessing that once the game has been put through its paces on the Wii U, there will be all sorts of new, advanced techniques to discuss. And this video doesn't cover things like the rage effect, which can definitely impact your matches. EDIT: and as some commenters are noting, it's entirely possible some of this can or will be patched out. Still, it's a good primer for many of the concepts that hardcore Smash players are knowledgeable about, and the sort of thing you'll want to be aware of while playing.